This invention relates to a method and an equipment for the purification of a liquid, wherein the liquid may be polluted by other liquids or solid material, e. g. removing oil from water, comprising a flotation device.
Extracting oil and gas offshore results in large amounts of sea water containing oil. The major part of the oil may be separated from the sea water in separating containers or tanks, but this purification is not sufficient for rendering the sea water back to the sea. Today, authoritative regulations say that spill water from offshore oil- and gas installations shall not contain more than 40 ppm oil.
When removing oil and/or other pollutants from water there are commonly a plurality of methods using centrifuges, hydrocyclones and flotation devices.
Centrifuges are advanced machines giving a high degree of purification, but their specific capacity is low. The use of a purification installation of this type when extracting oil, comprises a plurality of machines connected in parallel. Such installations are very expensive and it is required to keep a large stock of spare parts. The maintenance routines are also quite comprehensive.
Another disadvantage is that installations of this type are rather energy consuming. A typical energy consumption is 1 kwh/m.sup.3 of purified water.
The hydrocyclone may be considered as "common art" because they are preferred in new platform installations. These cyclones have quite good performances at favourable working conditions. Meanwhile, they have some limitations and disadvantages. In the presence of small oil droplets, a chemical flocculation device has to be installed upstream the hydrocyclone installation. The shear forces that may occur in the hydrocyclones are large, and the flakes have to be very robust. Flakes with the required strength may be difficult to produce.
Further, the pressure drop in the hydrocyclone is large, commonly more than 40 m W.P.
Downstream from the hydrocyclone it is required to have a flotation chamber with a rather large capacity when relieving the pressure. According to certain working conditions a hydrocyclone plant may be quite comprehensive to install. The capacities of the hydrocyclones are small, and therefore they have to be connected in groups to serve as large batteries when higher capacity is demanded.
The third type of separating devices, flotation plants, are characterised in that they have low specific capacities and low purification effects. The space needed for this type of installation is large, and they are often difficult to operate. Today, flotation plants are regarded as "technology of the past", and are of little relevance in offshore use because of the disadvantages these plants represent.